1. Field of the Invention
This invention relates to an apparatus for compensating convergence in a color cathode ray tube, more particularly to an apparatus for compensating the convergence drift of red and blue electron beams on a screen to improve convergence characteristics.
2. Description of the Prior Art
A color cathode ray tube is a device for producing an image on a screen with a light emission of a fluorescent material by landing an electron emitted from an electron gun and accelerated by a high voltage.
FIG. 6 shows a general structure of such a color cathode ray tube.
Electron beams are emitted from an In-Line type electron gun 2 mounted on the neck N of the color cathode ray tube. A convergence purity magnet (CPM) 4 is provided to converge the electron beams on the specific point on the screen and a deflection yoke is provided to scan the electron beams so that the desired picture is realized by landing the electron beams through a shadow mask on the corresponding red, green and blue fluorescent materials of the screen.
The CPM 4 comprises a magnetic ring assembly mounted on the neck having each pair of two, four and six-magnetic pole rings with a holder and a spacer so that the purity control by the two-magnetic pole rings and the convergence control by the four and six-magnetic pole rings are obtained.
The controls are performed as follows:
At first, the red(R), green(G) and blue(B) electron beams are deflected by a pair of two-pole rings so as to move their landing points in the same direction, whereby the landing position of the G beam is centered; second, the R and B beams are deflected by a pair of four-pole rings so that their landing points are moved in the opposite directions respectively, whereby the R and B beams are converged with the same distance; finally, the converged R and B beams are centered by a pair of six-pole rings so that the R, G and B electron beams are converged on the specific point on the screen.
In the case, an outer convergence variance(OCV) i.e., the distance between the R and B electron beams is controlled by the pair of four-pole rings and an center convergence variance(CCV) i.e., the distance between the G electron beam and a center position of the OCV is controlled by the pair of six-pole rings.
However, such a color cathode ray tube is on and some time passes, the thermal deformation is occurred in the electrodes of electron gun, which causes a convergence drift. That is, a first grid electrode(Gl electrode) 2b and a second grid electrode (G2 electrode) 2c adjacent to a heater of a cathode 2a are thermally expanded, thereby causing a thermal drift of convergence in the color cathode ray tube. The heater of the cathode 2a is generally heated at 780.degree. C., and the temperature of the first grid electrode 2b is raised up to 400.degree. C. and the second grid electrode 2c is heated to the temperature in the range of 150.degree. C. to 200.degree. C., whereby the initial convergence C is drifted to the positive direction (in the direction toward the electron gun from the screen), i.e., C1 as shown in FIG. 7 by the thermal expansion of the first grid electrode 2b, and the convergence is again drifted to the negative direction (in the direction opposite to the electron gun from the screen), i.e., C2 as shown in FIG. 7 by the thermal expansion of the second grid electrode 2c.
Accordingly, the convergence is drifted to about +0.05 mm (C1) for the initial 10-15 minutes of the operation of the color cathode ray tube under the thermal effect of the first grid electrode and then to -0.15 mm (C2) under the thermal effect of the second grid electrode after 120-150 minutes.
Also, in the conventional color cathode ray tube, a positive charge component is occurred in the end portion of the neck by the concentration of an impurity in the color cathode ray tube, which causes a charge drift(CG), so that the convergence is more drifted to -0.22 mm.
To compensate the convergence drift, Korean Utility Model Publication No. 96 -6527 discloses "Convergence magnet assembly with a dummy magnetic poles for cathode ray tube" in which a spacer formed with the magnetic poles PN and Ps of resin magnetic material is provided in the CPM to adjust the outer convergence variance and the center convergence variance.
However, such convergence magnet assembly cannot reduce the total convergence drift (C1+C2+CG).